a. Field of the Invention
The instant invention relates to capacitive coupling to aid ignition in discharge lamps.
b. Background
The popularity of thin, light-weight, wide screen televisions and computer monitors has resulted in tremendous interest and development of large screen liquid crystal display (LCD) televisions and monitors. The increase in LCD screen size has increased the demand for longer cold-cathode fluorescent lamps (CCFLs) and parallel architectures suitable for efficient drive of large CCFL arrays with high luminance uniformity and long life.
High frequency LCC resonant inverters, based on push-pull (Royer oscillators) or bridge (full or half) topologies are commonly used as electronic ballasts for powering single and dual CCFL backlighting systems. The resonant systems, generating sinusoidal waveforms of 25-100 KHz, have high losses associated with high frequency capacitive coupling, resonant circulating currents, and also cause luminance uniformity degradation due to the thermometer effect. As the number of lamps per system grows (some estimates are greater than forty), it may not be feasible to use an individual LCC drive for each lamp due to an increase in size, weight, cost, complexity of the enclosure design, and losses. While one solution is to drive multiple lamps with a single LCC ballast, such designs make it difficult to simultaneously maintain high efficiency, proper parallel lamp ignition, and individual lamp current regulation.
Lamp ignition, in particular, typically requires voltage levels significantly higher than the typical on-state operating voltage of the lamps due to a relatively high start-up impedance of the lamps, especially at DC or low-frequencies.